172 Anatomy Applied to Medicine and Surgery. 



together, or, to one or the other side, when the individual 

 muscle contracts, the trunk, at the same time, being rota- 

 ted to the opposite side. When the pelvis and spine are 

 both fixed, they act as muscles of expiration by compress- 

 ing the lower ribs, but, when the pelvis and thorax are 

 both fixed, they assist the diaphragm in compressing the 

 abdominal viscera. When the thorax alone is fixed, the 

 muscles, contracting, draw the pelvis upwards, as in climb- 

 ing. The different directions of the fibres of the external 

 and internal oblique, and the transversalis, give additional 

 strength to this region, thus lessening the possibility of a 

 hernia developing. When the recti muscles act from 

 above they flex the pelvis on the vertebral column, and, 

 when from below, they depress the thorax and flex the up- 

 per portion of the column, while the pyramidales render 

 tense the linea alba. 



3. Aponeuroses The aponeurosis of the in- 

 ternal oblique divides at the outer border of the 

 rectus muscle (excepting at its lower fourth), and 

 one layer passes in front and the other behind 

 the muscle, to unite at its inner border, in the linea 

 alba. In the lower fourth of the muscle there is no divi- 

 sion of the fascia, so that, the whole aponeurosis passes in 

 front of the rectus muscle. The aponeurosis of the ex- 

 ternal oblique, lying superficial to the preceding, blends 

 with it at the median line and assists in forming the linea 

 alba, while the aponeurosis of the transversalis muscle 

 passes behind the rectus with the posterior layer of the 

 aponeurosis of the internal oblique, except in the lower 

 fourth of the muscle, where it is placed in front. The 

 lower free margin of this posterior sheath of the rectus, 

 formed by the deep layer of the internal oblique aponeu- 

 rosis and by the transversalis aponeurosis, is termed the 

 fold of Douglas, and is arched slightly, so that its con- 



